This invention relates to branched polycarbonate compositions and a method of preparing them. More particularly, this invention relates to branched polycarbonate compositions having improved melt strength and a method of preparing them from aromatic cyclic polycarbonate oligomers in a melt equiliibration process.
Polycarbonate resins suitable for use in blow molding applications such as the production of hollow articles of large volume and large panels are required to have high melt strength (high shape retention) and high shear sensitivity characteristics such as a melt index ratio (MIR) greater than about 2.0 and a complex viscosity ratio (R*) greater than about 3.0. In order to achieve such properties, polycarbonates with a critical degree of branching are required.
It is known that branched polycarbonates, for example, bisphenol-A ("BPA") polycarbonates, which exhibit high melt strength properties and are thus suitable for blowmolding applications, can be prepared in a heterogeneous interfacial polymerization process. The use of polyhydric phenols having three or more hydroxy groups per molecule, for example, 1,1,1-tris-(4-hydroxyphenyl)ethane (THPE), 1,3,5-tris-(4-hydroxyphenyl)benzene, 1,4-bis-[di-(4-hydroxyphenyl)phenylmethyl]benzene and the like, as branching agents for high melt strength blow-moldable polycarbonate resins prepared interfacially has been described in U.S. Pat. Nos. Re. 27,682 and 3,799,953.
Other methods known to prepare branched polycarbonates through heterogeneous interfacial polymerization methods include the use of cyanuric, chloride as a branching agent (U.S. Pat. No. 3,541,059); branched dihydric phenols as branching agents (U.S. Pat. No. 4,469,861); and 3,3-bis-(4-hydroxyaryl)-oxindoles as branching agents (U.S. Pat. No. 4,185,009). Additionally, aromatic polycarbonates end-capped with branched alkyl acyl halides and/or acids and said to have improved properties are described in U.S. Pat. No. 4,431,793.
Trimellitic triacid chloride has also been used as a branching agent in the interfacial preparation of branched polycarbonate. Reference is made, for example, to U.S. Pat. Nos. Re 27,682 and 3,799,953, referred to above. Although trimellitic triacid chloride works well as a branching agent, the aromatic ester linkages formed may lead to yellowing, presumably due to photo-Fries reactions. Since much of the branched polycarbonate produced is used in packaging applications and twin wall sheets wherein clarity and avoidance of yellowing are important, alternatives have been sought to prepare a branched polycarbonate containing only carbonate linkages.
The interfacial polymerization processes described above are typically conducted in a mixed aqueous-organic system which results in recovery of the polycarbonate in the organic phase. Usually a dihydric phenol is reacted with a carbonate precursor in the presence of a chain terminating or molecular weight controlling agent wherein the branching agent is employed as a comonomer in the preparation of a branched polycarbonate.
In the interfacial processes presently used to prepare branched polycarbonates, the branching agent is used in an amount sufficient to obtain the proper melt rheological properties, i.e., R* of 3.6-3.9, for current applications. Increasing the level of branching agent and decreasing the level of capping agent could lead to a resin with a higher R* value. However, this may also lead to higher solution viscosities, which would effectively restrict the final molecular weight and other properties of the branched polycarbonates, and create problems of handling the product solutions. Adjusting the branching and endcapping levels to suit individual needs may cause problems in cross contamination and require excessive purging between different grades. It is desirable, therefore, to provide a more efficient means for varying the branching and endcapping levels according to need.
Blow-moldable grade, branched polycarbonates possessing properties of high melt strength and the like prepared in such interfacial polymerization methods, however, are typically specialty products which require a plant to stop making conventional grades of linear polycarbonate, thus hindering plant flexibility and increasing production costs in their manufacture. It is therefore desirable to provide an alternative method for preparing branched polycarbonates having high melt strength and shear sensitivity properties which does not involve interfacial polycondensation methods.
It is further desirable to provide a method of preparing branched polycarbonates having enhanced melt properties which make the branched polycarbonates useful in preparing relatively large parts in blow molding applications.